FR3111663A1 - GASOLINE THERMAL ENGINE VEHICLE AND CATALYST DIAGNOSIS METHOD - Google Patents

Abstract

A vehicle (V) comprises: - a petrol engine (MT) supplying combustion residues to an exhaust line (LE) comprising a first catalyst (CT1) treating combustion residues, a second catalyst (CT2) placed downstream of the first catalyst (CT1) and treating untreated combustion residues, and first (SO1) and second (SO2) probes measuring first and second values representative of the oxygen level upstream and downstream of the second catalyst ( CT2), and - a computer (CA) determining after rich and lean supply phases of the heat engine (MT), between first and second instants when the first and second values become lower respectively than first and second thresholds, a third value representative of an oxygen storage capacity of the second catalyst (CT2), and generating an alert when this third value is below a third threshold. Fig. 1

Description

VEHICLE WITH GASOLINE THERMAL ENGINE AND METHOD FOR DIAGNOSING CATALYSTS

Technical field of the invention

The invention relates to vehicles comprising a gasoline internal combustion engine and supplying an exhaust line with combustion residues, and more specifically to the diagnosis of the catalysts that certain exhaust lines of such vehicles include.

State of the art

Many vehicles include a powertrain (or GMP) comprising at least one thermal engine which consumes gasoline-type fuel to provide torque to move them and generates combustion residues which are treated in an exhaust line.

Some exhaust lines include a first catalyst treating the combustion residues generated, such as for example nitrogen oxides (or NO _x ) and/or unburned hydrocarbons (or HC) and/or carbon monoxide (or CO) , and a second catalyst placed downstream of the first catalyst and treating combustion residues not treated by the latter. Sometimes, these exhaust lines also include a particulate filter installed between their first and second catalysts and filtering by internal storage the soot particles forming part of the combustion residues and destroying these stored soot particles during regeneration phases.

Currently, there is a technique for diagnosing the first catalyst. This technique consists in installing, upstream and downstream of the first catalyst, first and second probes measuring respectively first and second values representative of the oxygen level, and in generating an alert when the variations of the second value (downstream) follow almost identically (with a slight time lag) the significant variations in the first value (upstream) of a rich phase supplying the thermal engine to a lean phase supplying the thermal engine, due to the significant variation in the richness of the air/fuel mixture. It will indeed be understood that when the second value varies relatively little from a rich phase to a lean phase, this means that the first catalyst performs its function correctly and is therefore in good condition.

Here, “wealth” means the air/fuel ratio.

But there is no technique to reliably diagnose the second catalyst. This is indeed complicated by the fact that the second catalyst acts in a similar way to the first catalyst but only on combustion residues which have not been treated by the latter, and therefore its action is more difficult to characterize. .

The aim of the invention is therefore in particular to improve the situation.

Presentation of the invention

It proposes in particular for this purpose a vehicle comprising a gasoline heat engine and supplying combustion residues to an exhaust line comprising a first catalyst treating combustion residues, a second catalyst placed downstream of the first catalyst and treating combustion residues not treated by the latter, and a first probe measuring a first value representative of a first oxygen level upstream of the second catalyst.

This vehicle is characterized by the fact:

- that its exhaust line also includes a second sensor measuring a second value representative of a second oxygen level downstream of the second catalyst, and

- that it also comprises a determining computer, after rich and lean phases of powering the thermal engine, between first and second instants when the first and second values become lower respectively than first and second thresholds, a third value representative of an oxygen storage capacity of the second catalyst, and generating an alert when this third value is below a third threshold.

Thanks to this diagnosis carried out on the second catalytic converter, we now know reliably that when the alert is generated, it no longer performs its function correctly and is therefore no longer in good condition.

The vehicle according to the invention may include other characteristics which may be taken separately or in combination, and in particular:

- its computer can be arranged in such a way as to trigger at selected times the rich phase of supply to the heat engine followed by the lean phase of supply to the heat engine, each rich phase lasting at least until the second value becomes greater than a fourth threshold, and/or each lean phase lasting at least until the first value becomes less than the first threshold;

- its computer can be arranged in such a way as to trigger each rich supply phase of the heat engine with a richness which is between 3% and 6%;

- its computer can be arranged in such a way as to trigger each rich supply phase of the heat engine with a richness equal to 4%;

- its exhaust line may include a particle filter between its first and second catalysts and filtering by internal storage of soot particles forming part of the combustion residues;

- the particulate filter can be installed between the first catalyst and the first probe;

- the first catalyst can be of the three-way type;

- the second catalyst can be of the three-way type;

- it can be of the automotive type.

The invention also proposes a diagnostic method intended to be implemented in a vehicle comprising a gasoline combustion engine and supplying combustion residues to an exhaust line comprising a first catalyst treating combustion residues, a second catalyst placed downstream of the first catalyst and treating combustion residues not treated by the latter, and a first probe measuring a first value representative of a first oxygen level upstream of the second catalyst.

This diagnostic method is characterized in that it comprises a step in which the realization by a second probe of measurements of a second value representative of a second oxygen level downstream of the second catalyst is triggered, and after rich and lean supply phases of the thermal engine, between first and second instants when the first and second values become lower respectively than first and second thresholds, a third value representative of an oxygen storage capacity of the second catalyst, and an alert is generated when this third value is below a third threshold.

The invention also proposes a computer program product comprising a set of instructions which, when it is executed by processing means, is capable of implementing a diagnostic method of the type of that presented above in a vehicle comprising a gasoline heat engine and supplying combustion residues to an exhaust line comprising a first catalyst treating combustion residues, a second catalyst placed downstream of the first catalyst and treating combustion residues not treated by the latter, and a first probe measuring a first value representative of a first oxygen level upstream of the second catalyst, to diagnose this second catalyst.

Brief description of figures

Other characteristics and advantages of the invention will appear on examination of the detailed description below, and of the appended drawings, in which:

schematically and functionally illustrates part of an embodiment of a vehicle according to the invention,

schematically illustrates within a diagram an example of curve (c1) of temporal evolution of the first value measured during a diagnosis, an example of curve (c2) of temporal evolution of the second value measured during this diagnosis , and an example of a curve (c3) of the temporal evolution of the fourth value measured during this diagnosis,

schematically and functionally illustrates an exemplary embodiment of a computer controlling the diagnostics of the catalysts of the exhaust line of the vehicle of FIG. 1, and

schematically illustrates an example of an algorithm implementing an example of a diagnostic method according to the invention.

Detailed description of the invention

The object of the invention is in particular to propose a vehicle V comprising a powertrain (or GMP) comprising at least one petrol engine MT and supplying combustion residues to an exhaust line LE including at least one (second) catalyst CT2 can be diagnosed.

In what follows, it is considered, by way of non-limiting example, that the vehicle V is of the automobile type. This is for example a car, as partially illustrated in Figure 1. But the invention is not limited to this type of vehicle. It relates in fact to any type of vehicle comprising a powertrain (or GMP) comprising at least one gasoline internal combustion engine and supplying an exhaust line with combustion residues. Thus, it concerns all land vehicles (commercial vehicles, motorhomes, minibuses, coaches, trucks, motorcycles, road construction machinery, construction machinery, agricultural machinery, leisure machinery (snowmobile, kart), and caterpillar machinery(ies) ), boats and aircraft.

There is schematically represented in FIG. 1 a part of a vehicle V, according to the invention, comprising a GMP comprising a petrol engine MT and supplying an exhaust line LE with combustion residues.

The thermal engine MT is supplied with gasoline-type fuel in order to provide torque to move the vehicle V, on the order of a supervision computer. In operation, this heat engine MT generates combustion residues which it delivers to an outlet S supplying an exhaust line LE.

In what follows and what precedes, the notions of upstream and downstream are used in reference to the direction of circulation of the combustion residues in the exhaust line LE.

This exhaust line LE comprises at least a first catalyst CT1, a second catalyst CT2, a first SO1 sensor, a second SO2 sensor and a CA computer.

The first catalyst CT1 is arranged so as to process combustion residues generated by the heat engine MT.

For example, the combustion residues treated by this first catalyst CT1 can be nitrogen oxides (or NO _x ) and/or unburnt hydrocarbons (or HC) and/or carbon monoxide (or CO).

Also, for example, this first catalyst CT1 can be of the “three-way” type. It is recalled that a catalyst is of the three-way type when the elements it contains (usually precious metals such as platinum, palladium or rhodium) cause three simultaneous reactions in the presence of nitrogen oxides, namely a reduction of these nitrogen oxides into dinitrogen and carbon dioxide (2NO + 2CO → N2 + 2CO2), an oxidation of carbon monoxide into carbon dioxide (2CO + O2 → 2CO2), and an oxidation of unburned hydrocarbons (or HC) into carbon dioxide and water (4CxHy + (4x+y)O2 → 4xCO2 + 2yH2O).

The second catalyst CT2 is placed downstream of the first catalyst CT1 and arranged so as to treat combustion residues which have not been treated by the first catalyst CT1.

For example, the combustion residues treated by this second catalyst CT2 can be nitrogen oxides and/or unburnt hydrocarbons and/or carbon monoxide.

Also, for example, this second catalyst CT2 can be of the “three-way” type.

The first SO1 sensor is installed upstream of the second catalyst CT2. It (SO1) is arranged so as to measure a first value v1 which is representative of a first oxygen level upstream of the second catalyst CT2. For example, this first value v1 can be a voltage.

For example, the first SO1 probe may be what those skilled in the art call a binary oxygen probe.

Preferably, the first sensor SO1 is installed just before the second catalyst CT2.

Note that this first SO1 probe can also be used to measure the oxygen level downstream of the first catalyst CT1.

The second SO2 sensor is installed downstream of the second catalyst CT2. It (SO2) is arranged so as to measure a second value v2 which is representative of a second oxygen level downstream of the second catalyst CT2. For example, this second value v2 can be a voltage.

For example, the second SO2 probe can be what those skilled in the art call an ammonia (or NH ₃ ) probe which also performs an oxygen level measurement function or even a binary oxygen probe.

The computer CA is responsible for checking the performance of at least the diagnostics of the second catalyst CT2. To this end, it (CA) is arranged in such a way as to determine, after rich and lean supply phases of the heat engine MT, between first and second instants when the first v1 and second v2 values become lower respectively than first s1 and second threshold s2, a third value v3 which is representative of an oxygen storage capacity of the second catalyst CT2. In addition, this computer CA is arranged so as to generate an alert when the third value v3 that it has determined is lower than a third threshold s3.

It will be understood that this alert is representative of the fact that the second catalyst CT2 no longer performs its function correctly and is therefore no longer reliably in good condition. This alert is intended for at least one user of the vehicle V and/or for a computer equipping the latter (V) and responsible for storing in a memory life information and in particular data representative of the problem detected on the second catalyst CT2. This data may, for example, be in the form of a specific code which can then be retrieved from an after-sales service when the vehicle V is brought. This memory, used for storing numerous vehicle life information V can, for example, be part of what those skilled in the art sometimes call the intelligent service unit (or BSI - supervision equipment) and/or of the CA computer diagnostic manager. The transmission by the computer CA to the memory of the alert (or of the alert message) then takes place via a communication network on board the vehicle V, and possibly multiplexed.

The user of the vehicle V can be alerted by displaying a dedicated message on at least one screen present in this vehicle V and/or broadcasting a dedicated message by at least one loudspeaker present in the vehicle V. display of the dedicated message can be done, for example, on the screen of the central instrument panel or of the dashboard of the vehicle V, or else on the screen of a communication device temporarily on board the vehicle V, able to communicate with at at least one computer of the latter (V), and accompanying a passenger, such as for example an intelligent mobile telephone (or “smartphone”) or a communicating electronic tablet. The sound broadcasting of the dedicated message can be done, for example, by at least one loudspeaker permanently equipping the vehicle V or else a loudspeaker equipping communication equipment temporarily on board the vehicle V.

For example, the third value v3 can be equal to the sum (or the integral) of the second values v2 determined between the first and second instants when the first v1 and second v2 values become lower respectively than first s1 and second s2 thresholds.

Preferably, the computer CA can be arranged in such a way as to trigger at selected times the rich phase of supply to the heat engine MT followed by the lean phase of supply to the heat engine MT. In this case, each rich phase lasts at least until the second value v2 becomes greater than a fourth threshold s4, and/or each lean phase lasts at least until the first value v1 becomes less than the first threshold s1. However, in a variant embodiment, it would be possible to envisage the duration of each rich phase being predefined and/or the duration of each lean phase being predefined.

In the presence of the last option, the computer CA can be arranged in such a way as to trigger each rich supply phase of the heat engine MT with a richness which is between 3% and 6%. For example, the computer CA can be arranged in such a way as to trigger each rich supply phase of the heat engine MT with a richness which is equal to 4%.

A richness of 4% means that the measurement of the oxygen level is representative of a value equal to 1.04. A voltage/richness correspondence table (in the case of a measurement by a binary probe) is used to identify the phases in which the richness corresponds to a value equal to 1.04. A current/richness correspondence table (in the case of a measurement by a proportional probe) is used to identify the phases in which the richness corresponds to a value equal to 1.04.

It will be noted, as illustrated without limitation in FIG. 1, that the exhaust line LE can also include a third SO3 probe installed upstream of the first catalyst CT1. It (SO3) is arranged so as to measure a fourth value v4 which is representative of a third oxygen level upstream of the first catalyst CT1 and which is used by the computer CA to carry out diagnostics of the first catalyst CT1. Indeed, the computer CA can be arranged in such a way as to determine, after rich and lean supply phases of the heat engine MT, between first and second instants when the fourth v4 and first v1 values respectively become lower than a fifth threshold s5 and at the first threshold s1, a fifth value v5 which is representative of an oxygen storage capacity of the first catalyst CT1.

For example, the third SO3 sensor can be a binary or proportional oxygen sensor.

In addition, the computer CA can also be arranged to generate an alert when the fifth value v5 that it has determined is below a sixth threshold s6.

It will be understood that this alert is representative of the fact that the first catalyst CT1 no longer performs its function correctly and is therefore no longer in good condition. As for the alert concerning the second catalyst CT2, this last alert is intended for at least one user of the vehicle V and/or for a computer fitted to the latter (V) and responsible for storing in a memory life information and in particular data representative of the problem detected on the first catalyst CT1. This data may, for example, be in the form of a specific code which can then be retrieved from an after-sales service when the vehicle V is brought.

The user of the vehicle V can be alerted by displaying a dedicated message on at least one screen present in this vehicle V and/or broadcasting a dedicated message by at least one loudspeaker present in the vehicle V.

For example, the fourth value v4 can be a voltage.

Also, for example, the fifth value v5 can be equal to the sum (or the integral) of the fourth values v4 determined between the first and second instants when the fourth v4 and first v1 values become lower respectively than the fifth s5 and first s1 thresholds.

There is schematically illustrated in FIG. 2, within a diagram, an example of curve c1 of temporal evolution of the first value v1 measured during a diagnosis of the second catalyst CT2, an example of curve c2 of temporal evolution of the second value v2 measured during this diagnosis, and an example of curve c3 of temporal evolution of the fourth value v4 measured during this diagnosis.

As can be seen, at a time t1 the computer CA triggers a rich supply phase of the heat engine MT and therefore the fourth value v4 (curve c3) quickly becomes greater than a predefined threshold (for example s4), representative of a rich mixture at the output S of the heat engine MT. Then, at a time t2 (> t1) the first value v1 (curve c1) becomes greater than a predefined threshold (for example s4), representative of a rich mixture downstream of the first catalyst CT1. Then, at a time t3 (> t2) the second value v2 (curve c2) becomes greater than a predefined threshold (for example s4), representative of a rich mixture downstream of the second catalyst CT2. The computer CA then decides to stop the rich phase at time t4 (> t3) and to trigger the lean phase. At a time t5 (> t4) the fourth value v4 (curve c3) becomes lower than the fifth threshold s5 (representative of a lean mixture at the output S of the thermal engine MT), and therefore the computer CA begins to determine the fifth value v5 . Then, at a time t6 (> t5) the first value v1 (curve c1) becomes lower than the first threshold s1 (representative of a lean mixture downstream of the first catalyst CT1), and therefore the computer CA has the fifth value v5 final which it can compare to the sixth threshold s6, puts an end to the lean phase, and begins to determine the third value v3. If the fifth value v5 is lower than the sixth threshold s6, the computer CA generates a dedicated alert (concerning the first catalyst CT1). At a time t7 (> t6) the second value v2 (curve c2) becomes lower than the second threshold s2 (representative of a lean mixture downstream of the second catalyst CT2), and therefore the computer CA has the third final value v3 that 'it can compare to the third threshold s3. If the third value v3 is lower than the third threshold s3, the computer CA generates a dedicated alert (concerning the second catalyst CT2).

It will also be noted, as illustrated without limitation in FIG. 1, that the exhaust line LE can also comprise a particulate filter FP installed between its first catalysts CT1 and second CT2 and arranged so as to filter soot particles by internal storage. which are part of the combustion residues. For example, and as illustrated without limitation in FIG. 1, this particulate filter FP can be more precisely installed between the first catalyst CT1 and the first probe SO1. But it (FP) could also be installed between the first probe SO1 and the second catalyst CT2.

Preferably, this particle filter FP is also arranged so as to destroy the soot particles stored during regeneration phases (possibly triggered by the computer CA). There may also be spontaneous passive regeneration when the driver takes their foot off the accelerator pedal and in the presence of high temperature.

It will be noted that the first s1, second s2 and fifth s5 thresholds may optionally be identical, as in the example illustrated without limitation in FIG. 2. But this is not compulsory. Indeed, they (s1, s2 and s5) could be different from each other.

It should also be noted that the CA computer can be dedicated to diagnostics of the LE exhaust line. But in a variant, it may be a computer also performing other functions than the diagnostics of the exhaust line LE, such as for example the supervision computer which is suitable for controlling (or supervising) the power supply and the operation of at least the heat engine MT.

In order to ensure its functions (and therefore its operations), the computer CA comprises at least one processor PR, for example of digital signal (or DSP (“Digital Signal Processor”)), and at least one memory MD. The processor PR can comprise integrated (or printed) circuits, or else several integrated (or printed) circuits connected by wired or wireless connections. By integrated (or printed) circuit is meant any type of device capable of performing at least one electrical or electronic operation. The memory MD is live in order to store instructions for the implementation by the processor PR of at least part of the diagnostic method described below. The CA computer can therefore be produced in the form of a combination of electrical or electronic circuits or components (or “hardware”) and software modules (or “software”).

It will also be noted, as illustrated without limitation in FIG. 3, that the computer CA can also comprise (in addition to its random access memory MD and its processor PR), a mass memory MM, in particular for storing the first v1, second v2 and fourth v4 measured values, and intermediate data involved in all its calculations and processing. Furthermore, the calculator CA can also comprise an input interface IE for receiving at least the first v1, second v2 and fourth v4 measured values, to use them in calculations or processing, possibly after having formatted them and/or demodulated and/or amplified, in a manner known per se, by means of a digital signal processor PR′. In addition, the computer CA can also include an output interface IS, in particular to deliver its dedicated messages and commands (possibly to trigger and interrupt the rich phases and the lean phases during the diagnostics).

The invention can also be considered in the form of a diagnostic method intended to be implemented in the vehicle V. This diagnostic method can be implemented at least partially by the computer CA described above.

This diagnostic method comprises a step 10-100 in which one (the computer CA) begins by triggering the production by the second SO2 probe of measurements of the second value v2 which is representative of the second oxygen level downstream of the second catalyst CT2 . Then, this step 10-100 continues with the determination (by the computer CA) after rich and lean supply phases of the heat engine MT, between first and second instants when the first v1 and second v2 values respectively become lower than the first s1 and second s2 thresholds, a third value v3 which is representative of an oxygen storage capacity of the second catalyst CT2. Then, this step 10-100 continues with the generation (by the computer CA) of an alert when the third value v3 is lower than the third threshold s3.

Schematically illustrated in FIG. 4 is an example of an algorithm implementing an example of a diagnostic method according to the invention.

The algorithm includes a sub-step 10 in which one (the computer CA) triggers a rich phase of powering the heat engine MT.

Then, in a sub-step 20, it (the computer CA) determines whether the second value v2 is greater than a predefined threshold (for example s4). If not (v2<s4), one (computer CA) performs sub-step 20 again. On the other hand, if yes (v2>s4), one (computer CA) stops the rich phase and triggers a lean power phase of the heat engine MT in a sub-step 30.

Then, in a sub-step 40, it (the computer CA) determines whether the first value v1 is lower than the first threshold s1. If not (v1>s1), one (computer CA) again performs sub-step 40. On the other hand, if yes (v1<s1), one (computer CA) stops the lean phase and determining the third value v3 in a sub-step 50.

Then, in a sub-step 60, it (the computer CA) determines whether the second value v2 is lower than the second threshold s2. In the negative (v2>s2), one (the computer CA) performs the sub-step 60 again. On the other hand, in the affirmative (v2 <s2), one (the computer CA) obtains the third final value v3 in a sub-step 70.

Then, in a sub-step 80, it (the computer CA) determines whether the third value v3 is lower than the third threshold s3. If not (v3>s3), one (computer CA) ends the algorithm in a sub-step 90. On the other hand, if yes (v3<s3), one (computer CA) generates an alert dedicated to the second catalyst CT2.

It will also be noted that the invention also proposes a computer program product (or computer program) comprising a set of instructions which, when it is executed by processing means of the electronic circuit (or hardware) type, such as for example the processor PR, is capable of implementing the diagnostic method described above for diagnosing the second catalyst CT2 of the exhaust line LE of the vehicle V.

It will also be noted that one or more substeps of the step of the diagnostic method can be carried out by different components. Thus, the diagnostic method can be implemented by a plurality of digital signal processors, random access memory, mass memory, input interface, output interface.

Claims (10)

Vehicle (V) comprising a gasoline engine (MT) and supplying combustion residues to an exhaust line (LE) comprising i) a first catalyst (CT1) treating combustion residues, ii) a second catalyst (CT2) placed downstream of said first catalyst (CT1) and treating combustion residues not treated by the latter (CT1), and iii) a first probe (SO1) measuring a first value representative of a first oxygen level upstream of said second catalyst (CT2), characterized in that said exhaust line (LE) comprises a second probe (SO2) measuring a second value representative of a second oxygen level downstream of said second catalyst (CT2), and in that it further comprises a computer (CA) determining after rich and lean supply phases of said heat engine (MT), between first and second instants when said first and second values become lower respectively than first and second thresholds, a three th value representative of an oxygen storage capacity of said second catalyst (CT2), and generating an alert when said third value is below a third threshold. Vehicle according to claim 1, characterized in that said computer (CA) is arranged in such a way as to trigger at chosen instants said rich phase of supply of said heat engine (MT) followed by said lean phase of supply of said heat engine (MT ), each rich phase lasting at least until said second value becomes greater than a fourth threshold, and/or each lean phase lasting at least until said first value becomes less than said first threshold. Vehicle according to Claim 2, characterized in that the said computer (CA) is arranged so as to trigger each rich supply phase of the said heat engine (MT) with a richness of between 3% and 6%. Vehicle according to Claim 3, characterized in that the said computer (CA) is arranged so as to trigger each rich supply phase of the said heat engine (MT) with a richness equal to 4%. Vehicle according to one of Claims 1 to 4, characterized in that the said exhaust line (LE) comprises a particle filter (FP) between the said first (CT1) and second (CT2) catalysts and filters by internal storage of the particles soot forming part of said combustion residues. Vehicle according to Claim 5, characterized in that the said particulate filter (FP) is installed between the said first catalyst (CT1) and the said first probe (SO1). Vehicle according to one of Claims 1 to 6, characterized in that the said first catalyst (CT1) is of the three-way type. Vehicle according to one of Claims 1 to 7, characterized in that the said second catalyst (CT2) is of the three-way type. Diagnostic method for a vehicle (V) comprising a petrol engine (MT) and supplying combustion residues to an exhaust line (LE) comprising i) a first catalyst (CT1) treating combustion residues, ii) a second catalyst (CT2) placed downstream of said first catalyst (CT1) and treating combustion residues not treated by the latter (CT1), and iii) a first probe (SO1) measuring a first value representative of a first rate of oxygen upstream of said second catalyst (CT2), characterized in that it comprises a step (10-100) in which the production by a second probe (SO2) of measurements of a second value representative of a second rate is triggered of oxygen downstream of said second catalyst (CT2), and after rich and lean supply phases of said heat engine (MT), it is determined between first and second instants when said first and second values become lower respectively than first and second e thresholds, a third value representative of an oxygen storage capacity of said second catalyst (CT2), and an alert is generated when said third value is below a third threshold. Computer program product comprising a set of instructions which, when it is executed by processing means, is capable of implementing the diagnostic method according to claim 9 in a vehicle (V) comprising a heat engine (MT ) gasoline and supplying combustion residues to an exhaust line (LE) comprising i) a first catalyst (CT1) treating combustion residues, ii) a second catalyst (CT2) placed downstream of said first catalyst (CT1) and treating combustion residues not treated by the latter (CT1), and iii) a first probe (SO1) measuring a first value representative of a first oxygen level upstream of said second catalyst (CT2), to diagnose this second catalyst (CT2).